The invention relates generally to locking nuts for use with a spinal implant system, and more particularly to a self-aligning cap nut, which requires less clearance for tightening and further which has improved rod gripping characteristics.
It has become an established procedure in treating certain spinal conditions to surgically implant a spinal stabilization system. Generally, these systems include two similar sets of implants, which are inserted on both sides of the spinous processes. Each set has one or more elongated rods, or plates, which span multiple vertebral bodies and secure them in alignment by means of a bone anchor member, typically a screw or a vertebral hook. The anchor may include a channel, which is intended to cradle the rod as it extends along its longitudinal axis form one anchor to the next. Finally, locking means secure the rod in the channel so that its position relative to the anchor remains fixed. The locking mechanisms include sliding members, which form a dovetail cooperation with a top opening in the anchor. Other locking members may include a threaded cooperation with the anchor, which allows the locking member to be tightened down into a secure contact with the rod.
As these implant systems may be used to correct a misalignment of the vertebral bodies, relatively large forces may act against the components. For example, the rod may be subjected to torque and/or bending, which the anchor and locking member must counter in cooperation so that the system may function as intended. Particularly strong forces are generated when a surgeon uses the system for correction of a spinal curvature, such as scoliosis.
In addition to the issues regarding the ability of the system to resist forces, these systems must be as minimal and non-invasive as possible in order to inhibit negative impact on the affected surgical area. Further, the system must be designed for ease of implantation and assembly. It is an advantage for the system to be streamlined with few parts for assembly, and with parts that go together easily and which can be disassembled as well.
The current invention provides a locking member, which can be used in conjunction with externally threaded bone anchors currently in existence. However, in order to reduce the space needed to apply the new locking member, it has an internal torque driving surface. In particular, this internal surface is a hexagonal recess, although it is envisioned that other torque receiving configurations could be used. Further, the locking member of the present invention is self-aligning so as to minimize cross threading. This is accomplished by providing an internal projection or boss, which extends into the recess of the anchor member and fills the recess above the rod channel between the sidewalls of the anchor. This projection also acts to stabilize the sides of the anchor so as to prevent the side-walls from collapsing inward at high torque forces. In addition, the projection includes a high friction surface on the bottom in order to further increase the gripping force which can be applied to the rod by the locking member or cap nut. In particular, this high friction surface is a circular ridge, which provides for two additional points of contact with the rod surface. Thus, the cap nut of the present invention grips the rod with the external edge of the nut and with the internal ridges of the projection, which contact the rod.